Secrecy communication system



Dec. 24, 1946. P. K. cHATTl-:RJEA ET AL 2,412,954

SEGRECY COMMUNICATION SYSTEM Filed Aug. 28, 1945l Inventors Patented Dec. 24, 1946 ,insel SECRECY COMMUNICATION SYSTEM E'rafulla Kumar Chatterjea and Leslie Wilfred Houghton,

London, England,

assignors to Standard Telephones and Cables Limited, London, England, a British company Application August 28, 1943, Serial No. 500,416 In Great Britain October 23, 1942 Cl. Z50-17) 8 Claims. l

The present invention relates to an electrical transmission system for secret communication in which the secret intelligence is masked by the transmission of other signals, which may or may not convey intelligence, in such a way that the existence of the secret communication is hidden.

'Ihe invention is a modification of the pulse transmission system described and claimed in the specification of our copending application No. 468,573 led December 10, 1943, and is for the purpose of avoiding certain practical diculties which have been experienced in meeting the requirements of the system.

In this system, the signal wave actually transmitted (which may, for example, be used to modulate a radio transmitter or carrier frequency system according to known principles) comprises a mixture of a masking wave and a series of regularly repeated short duration pulses which are amplitude modulated by the secret signal. The signals conveyed by the masking wave may be received in any ordinary manner, and the presence of the pulses -conveying the secret information is not suspected. These pulses can only be rendered evident by the use of the special receiving arrangements described in the specication referred to.

It is found, however, that although the mixing arrangements at the transmitting end produce a combined wave in which the pulses are theoretif cally obscured in the manner explained, in practice, owing to the imperfections of the circuits and apparatus used, it is difficult to prevent the appearance of pulses which will betray the presence of the secret signal. The present invention is concerned with an improved arrangement at the transmitting end which produces the same type of mixed signal wave without the liability to generate pulses which betray the secret signal. The receiving arrangements may be the same as those described in the above mentioned specification.

According to the invention there is provided a secrecy communication system in which the ntelligence to be communicated is transmitted as a, composite electrical wave comprising a masking Wave periodically interrupted for short equal time intervals for the insertion therein of the individual pulses of a pulse train amplitude modulated in accordance with the intelligence wave, the pulses having the same period and duration as the interruptions; provided with a transmitting arrangement comprising means for adding togather the masking wave and the intelligence wave to produce a mixed Wave, means to generate a train of short regularly repeated pulses, means to modulate the amplitude of the pulses in accordance with the mixed wave and means for adding the masking wave to the modulated pulses with such amplitude, sense, and phase'that the proportion of the modulation of the pulses which was due to the masking wave is removed.

The invention will be described with reference to the accompanying drawing in which:

Figure l shows a `block schematic diagram of agtransmitting arrangement according to the invention;

Figures 2 and 3 show explanatory waveform diagrams; and Figure 4 shows a, schematic circuit diagram of a pulse modulator according to the invention.

By reference to the specification describing the original invention it Will be seen that in the transmitter a train of equally spaced short pulses is amplitude modulated by the secret signal. At the same time the amplitude of a suitable masking signal wave is reduced periodically to zero for successive intervals each equal to the duration of the pulses, and the amplitude modulated pulses are inserted in these intervals, so that the pulses in the composite wave cannot be perceived by ordinary methods. The success of this operation depends'on the accurate fitting of the modulated pulses in the spaces cut out of the masking signal Wave, otherwise pulses which may be easily detected will be produced. This has been found difiicult to arrange and according to the present invention the composite wave is produced by iirst adding together the masking signal and the secret signal waves and using the product to modulate the amplitude of the pulse train. The masking signal wave is then mixed with the modulated pulse train in such a sense that the masking components of the pulse amplitude modulation are balanced out, leaving only the modulation due to the secret signal, while in the intervals between the pulses the masking signal wave is present normally as before. This avoids the necessity of fitting the pulses into the periods cut out of the wave by a separate operation.

Figure 1 shows a block schematic circuit diae gram of a transmitting arrangement according to the present invention. An oscillator l supplies a frequency fp to a pulse generator 2 from which areobtained short pulses having a duration small compared with the intervals between the pulses, the frequency of repetition being fp. The oscillator l also supplies the frequency fp to a source 3 of a masking signal which may be an amplifier to which is supplied a signal at MS, o r it may be,`

a device in which any suitable signal is generated.

The secret signal which is to be masked is supplied at SS to an amplifier 5 and thence to a device 6 where it is added to the masking signal wave which already contains the frequency fp which is added ior demodulation purposes at the receiving end, as described in the previously mentioned speciiication. For convenience, the output of the device 3 will be called the masking signal, in what follows. The mixed signal wave from S is supplied at b to a pulse modulator i for the purpose of amplitude modulating the pulses generated in the device 2, which are supplied to the modulator fi at a.. rIhe output c is connected to an adding device 'I, which is also supplied with the masking signal wave from 3, The output of 'i is then fed to a radio-transmitter or other translating device represented by the block 3.

The pulses obtained at c are amplitude modulated by a composite wave comprising a mixture of the masking signal and the secret signal waves.

Figure 2 shows a series of such pulses, the dotted outline representing the above mentioned composite Wave. The masking signal wave is added to the modulated pulses in the device l with such an amplitude, sense, and phase that the proportion of the amplitude modulation which was due to the masking signal is neutralised, so that only the modulation due to the secret signal remains. This is indicated in Figure 3, in which the outline of thefull-line curve represents the masking signal wave which when added to the pulses of Figure 2 causes their tips to follow the dotted line curve of Figure 3, which is the curve corresponding to the secret signal. 'The wave obtained at the output d of the device 'I is the full line curve of Figure 3, which comprises the masking signal interrupted bythe pulses whose tips denne the secret signal. This is the same type of wave as is provided at the output of the adding tailed schematic circuit diagram of a preferred L* form of the pulse modulator 4 is shown in Figure 4. This circuit comprises four thermionic amplifying valves of any suitable type (shown as triodes for clearness), designated V1, V2, V3 and V4. to the invention which will be fully described, the circuit also includes conventional operating arrangementsfor the valves well known in the art; and of these A indicates an anode decoupling resistance, G a grid leak resistance, Q a cathode bias resistance, C a by-pass condenser and K a blocking condenser, these all having appropriate values. B represents a suitable resistance for providing cathode bias from the anode potential source I-IT-i- Which is connected to terminal I8.

The input terminals for the pulses to be modulated are I II and I2, those for the modulating signal are i3 and Hi, and the output terminals for the modulated pulses are i5 and I6, These pairs of terminals are respectively marked a, b and c to correspond with the markings applied to the pulse modulator i in Figure 41.

The valves V1 and V2 should be as closely alike as possible, and form one pair, while V3 and V4 should also be alike and form another pair; but they need not be the same as V1 and V2. The valves V1 and V2 are respectively provided with equal anode load resistances R1 and Rz and equal cathode load resistances R3 and R4. The control grids of these two valves are supplied in parallel Besides the special arrangements according with unidirectional pulses (for example positive pulses) from the pulse generator 2 (Fig. 1) through the terminals II and I2. The resistances R1 and R2 being earthed for the signals through the corresponding by-pass condensers C, equal negative pulses will be obtained 'from the anodes oi V1 and V2; and also equal positive pulses Will be obtained from the resistances R3 and R4. The resistances should be so chosen that the positive pulses obtained across R4 are equal in amplitude to the negative pulses obtained across R1.

The control grids of the valves V3 and V4 are connected respectively to R1 and R4 as shown so that one of them is energized positively and one negatively. As a result the drop across the common anode rload resistance R5 remains constant, and there will be no output at the terminals I5 and I6.

Owing to the phase reversal in the valves, V3 will produce a negative pulse at the output terminals l5 and iii, which is exactly neutralized by the positive pulse produced by Vi, and there will,

herefore, be no output pulses. However, when the mixed signal wave from the device 5 (Fig. 1) is applied to the terminals I3 and I4, the anode potential of V2 will be modulated. When the instantaneous amplitude of the signal wave applied to terminals i3 and I4 is positive, the positive pulses applied to Va will be increased inamplitude and will, therefore, preponderate over the negative pulses applied to V4, and accordingly negative output pulses will be obtained at terminals I5 and i6 having amplitude equal to the difference between the amplitudes of the applied pulses, and proportional to the amplitude of the applied signal wave. When the instantaneous amplitude of the latter is negative the output pulses will obviously be positive. The Voutput at terminals I5 and I6 will accordingly be of .the type shown in Figure 2.

The valves V1 and V2 ,should bebiased beyond the cut-0E, and this will prevent them from being operated during ythe intervals 4between .the pulses. `The anode circuits are decoupled by means of the condensers .C in order to yprevent the modulating signal wave from aiecting the anode voltage of V1.

`It will be evident that positive output pulses could be obtained by applying the modulating signal wave to V1 instead of to V2, or by supplying an eXtra reversing stage of amplication (not shown), or by supplying negative pulses from the pulse generator 2 (Figure 1) instead of positive pulses.

What is claimed is:

1. A secrecy communication system in which the intelligence to be communicated is transmitted as a composite `electrical wave comprising a masking wave periodically interrupted for short equaltime intervals for the insertion therein of the individual pulses of a pulse train amplitude modulated in accordance with the intelligence wave, the pulses having the same period andduration as the interruptions; provided with a transmitting arrangement comprising means for adding together the masking wave and the intelligence wave to produce a mixed wave, lmeans to generate a train of short regularly repeated pulses, means to modulate the amplitude of the pulses in accordance with the mixed wave .and means for adding the masking wave tothe modulated pulses with such amplitude, sense, and phase that the proportion of the modulation of the pulses which was clue .to the .masking wave Yis removed.

2. A system according to claim 1 comprising a' pulse generator controlled by an oscillator for producing the regularly repeated pulses, the masking wave being adapted to contain a sinusoidal wave component derived from the oscillator and having a frequency equal to the pulse repetition frequency.

3. A system according to claim l in which the modulating means comprises a first pair of similar thermionic valves and means to apply the regularly repeated pulses simultaneously to the control grids of the two valves, each valve being provided with an output load resistance so disposed that pulses of equal amplitudes but of opposite signs are produced across the two load resistances, respectively.

4. A system according to claim l in which the modulating means comprises a rst pair of similar thermionic valves, means to apply the regularly repeated pulses simultaneously to the control grids of said two valves, a rst load resistance connected in series with the anode of one valve and a second load resistance connected in series with the cathode of the other valve.

5. A system according to claim 1 in which the modulating means comprises a first pair of similar thermionic valves, means to apply the regularly repeated pulses simultaneously to the control grids of said valves, a first load resistance connected in series with the anode of one of said valves, a second load resistance connected in series with the cathode of the other of said valves and a second pair of similar thermionic valves having their anodes connected through a common resistance to the source of anode potential and their control grids connected respectively to said rst and second load resistances.

6. A system according to claim 1 in which the 6 modulating means comprises a first pair of similar thermionic valves, means to apply the regularly repeated impulses simultaneously to the control grids of said two valves and means for applying said masking signal wave to the anode of one of said valves.

7. A system according to claim 1 in which the modulating means comprises a first pair of similar thermionic valves, means to apply the regularly repeated pulses simultaneously to the control grids of said two valves, a first load resistance connected in series with the anode of one of said valves, a second load resistance connected in series with the cathode of the second said valve, a second pair of similar thermionic valves having their anodes connected through a common resistance to the source of anode potential and their control grids connected respectively to the two load resistances and an output circuit for obtaining modulated output pulses from the anodes oi the said second pair of valves.

3. A system according to claim l in which the modulating means comprises a rst pair of similar thermionic valves, means for biasing said valves beyond the cut-off point, means to apply the regularly repeated pulses simultaneously to the control grids of said two Valves, a rst load resistance connected in series with the anode of one of said valves, a second load resistance connected in series with the cathode of the other of said valves, and a second pair of similar thermionic valves having their anodes connected through a common resistance to the source of anode potential and their control grids connected respectively to said first and second load resistances.

PRAFULLA KUMAR CHATTERJEA. LESLIE WILFRED HOUGHTON. 

